1. CRP Structure and Activity
C-reactive protein was first described by Tillett and Francis [J. Exp. Med., 52:561-71 (1930)] who observed that sera from acutely ill patients precipitated with the C-polysaccharide of the cell wall of Streptococcus pneumonia. Other investigators subsequently identified the reactive serum factor as protein, hence the designation "C-reactive protein" or "CRP." Kilpatrick et al., Immunol. Res., 10:43-53 (1991), provides a recent review of CRP.
CRP is a pentameric molecule which consists of five identical subunits [Osmand et al., Proc. Natl. Acad. Sciences, U.S.A., 74:739-743 (1977)]. This pentameric form of CRP is sometimes referred to as "native CRP."
The gene sequence for human CRP has been cloned [Lei et al., J. Biol. Chem., 260:13377-13383 (1985)]. In addition, the primary sequences for rabbit CRP [Wang et al., J. Biol. Chem., 257:13610-13615 (1982)] and murine CRP have been reported [Whitehead et al., Biochem. J., 266:283-290 (1990)], and is under investigation for rat, dog, horse, goat, and sheep. Clinical and laboratory observations have determined that the acute phase response, classically defined by the well-defined changes of the blood [Pepys et al., Advances in Immunology, 34:141-212 (1983)], develops during various states of disease and injury including malignant neoplasia, ischemic necrosis, and bacterial, viral, or fungal parasitic infections. Measurement of serum acute phase reactants such as CRP have been utilized in clinical tests for diagnosis and clinical management of patients with various conditions, including systemic lupus erythematosus (SLE) [Bravo et al., J. Rheumatology, 8:291-294 (1981)], rheumatoid arthritis [Dixon et al., Scand. J. Rheumatology, 13:39-44 (1984)], graft versus host disease [Walker et al., J. Clin. Path., 37:1022-1026 (1984)], as well as many other diseases.
2. Modified-CRP Structure and Activity
In about 1983, another form of CRP was discovered which is referred to as "modified C-reactive protein" or "mCRP." mCRP has significantly different charge, size, solubility and antigenicity characteristics as compared to native CRP [Potempa et al., Mol. Immunol., 20:1165-75 (1983)]. mCRP also differs from native CRP in its binding characteristics. For instance, mCRP does not bind phosphorylcholine [Id.; Chudwin et al., J. Allergy Clin. Immunol., 77:216a (1986)].
The distinctive antigenicity of mCRP has been referred to as "neo-CRP." Neo-CRP antigenicity is known to be expressed on:
1) CRP treated with acid, urea or heat under certain conditions;
2) the primary translation product of DNA coding for human and rabbit CRP; and
3) CRP immobilized on plastic surfaces [Potempa et al., Mol. Immunol., 20:1165-75 (1983); Mantzouranis et al., Ped. Res., 18:260a (1984); Samols et al., Biochem. J., 227:759-65 (1985); Potempa et al., Mol. Immunol., 24:531-541 (1987)]. A molecule reactive with polyclonal antibody specific for neo-CRP has been identified on the surface of 10-25% of peripheral blood lymphocytes (predominantly NK and B cells), 80% of monocytes, and 60% of neutrophils, and as well as at sites of tissue injury [Potempa et al., FASEB J., 2:731a (1988); Bray et al., Clin. Immunol. Newsletter, 8:137-140 (1987); Rees et al., Fed. Proc., 45:263a (1986)].
Furthermore, mCRP differs from native CRP in its biological activity. It has been reported that mCRP can influence the development of monocyte cyto-toxicity, improve the accessory cell function of monocytes, potentiate aggregated IgG-induced phagocytic cell oxidative metabolism, and increase the production of interleukin-1, prostaglandin E and lipoxygenase products by monocytes [Potempa et al., Protides Biol. Fluids, 34:287-290 (1987); Potempa et al., Inflammation, 12:391-405 (1988); Chu et al., Proc. Amer. Acad. Cancer Res., 28:344a (1987); Potempa et al., Proc. Amer. Acad. Cancer Res., 28:344a (1987); Zeller et al., Fed. Proc., 46:1033a (1987); Chu et al., Proc. Amer. Acad. Cancer Res., 29:371a (1988)].
In vivo experiments with mCRP were performed to determine if mCRP was capable of providing a protective effect against lethal doses of Streptococcal pneumonia [Chudwin et al., J. Allergy Clin. Immunol., 77:216a (1986)]. These studies demonstrated that intravenous administration of mCRP not only protected the animals from lethal S. pneumonia doses but that mCRP efficacy was 3 to 4 fold greater than native CRP.